Cytomegalovirus and transcriptional down-regulation of major histocompatibility complex class II expression.

CD4(+)T lymphocytes are a significant component of the afferent and efferent arms of adaptive immunity and are critical for controlling viral infections. CD4(+)T lymphocytes secrete cytokines that augment CD8(+)T lymphocyte and B lymphocyte responses and directly inhibit viral replication. The interface between the CD4(+)T lymphocyte and virus is the major histocompatibility complex (MHC) class II molecule. Cytomegalovirus, a beta-herpesvirus, has evolved mechanisms for inhibiting MHC class II expression and thus escaping CD4(+)T lymphocyte immunosurveillance. Herein, we review cytomegalovirus-mediated down-regulation of inducible and constitutive MHC class II expression, while focusing on lesions that occur at the level of MHC class II transcription.